Low Noise Amplifiers (LNAs) are an integral part of many high performance receivers and their design in the literature with noise figures <3 dB has typically been limited to the use of source-degenerated LNAs and noise-cancelling LNAs. While the former provide impressive noise figure numbers using two to three integrated inductors in a CMOS technology, they suffer from degraded linearity from the passive gain in the input matching network. Noise-cancelling LNAs, on the other hand, typically achieve comparable noise figures with linearity, albeit at the cost of dramatically high power consumption spent in realizing the noise-cancelling network by active means.
FIG. 1 shows an example 100 of a typical block diagram of a noise cancelling LNA circuit using a “common-gate (CG) matching network.” As illustrated, circuit 100 may include a common gate matching network 108, a drain resistor Rd 114, a source resistor Rs 116, a first amplifier 120, a second amplifier 118, and an adder 122.
As also shown in FIG. 1, any suitable antenna/source 102 can provide an input signal to the LNA circuit, and antenna/source 102 may be represented as a voltage source 104 and an output resistance 106.
As used herein, the term “common-gate matching network” refers to the broad class of LNAs using a common gate input stage with or without gain boosting at the gate.
The noise from the CG matching network can be modeled as a current source (in) 124 as shown in FIG. 1. It can be shown that the transfer function of in has a null when the gains A1 of amplifier 120 and A2 of amplifier 118 are related as A1/A2=a(1+R0/RS)/(1−R0/RS), where a is the gain of the CG matching network and, when the LNA is matched to its input, a=Rd(1−R0/Rs)/Rs. The realization of the gains A1 and A2 is typically done by active means and hence the noise and distortion characteristics of the LNA are ultimately limited by the linearity and noise of the amplifiers, A1 and A2.